1. Field of the Invention
The invention relates to a control apparatus for an internal combustion engine.
2. Description of the Related Art
Conventionally, auxiliaries mounted in a vehicle such as an air-conditioner, a power steering device, and a headlamp are directly or indirectly driven by the output from an internal combustion engine for a vehicle. When such an internal combustion engine is running at a low speed, for example, idling, the amount of air taken in a combustion chamber is increased if it is determined that an auxiliary is turned on. Increasing the intake air amount in this manner suppresses reduction in the engine speed that is likely to occur due to an increase in the engine load caused by turning on the auxiliary.
There is a conventional technology in which the intake air amount is increased by adjusting the opening amount of a throttle valve arranged in an intake air passage. With this technology, however, a non-negligible time-lag is caused between when the throttle valve is controlled and when the amount of air in the combustion chamber is actually increased.
Japanese Patent Application Publication No. 2005-232992 (JP-A-2005-232992) describes a technology in which a variable valve mechanism that adjusts the duration of an intake valve is provided in an internal combustion engine, and the amount of air taken in a combustion chamber is increased by the variable valve mechanism. In this internal combustion engine, the intake air amount is increased by controlling the variable valve mechanism to extend the duration of the intake valve. As a result, the amount of air actually taken in the combustion chamber is rapidly increased.
If the duration of the intake valve is extended, the period during which the intake valve is open is prolonged. Accordingly, the period during which both the intake valve and an exhaust valve are open, namely, the valve overlap time is also prolonged. If the valve overlap time is prolonged, the following non-negligible inconveniences are caused.
Because the air flowing through the intake passage pulsates, the pressure in the intake passage constantly changes. Accordingly, if the valve overlap time is prolonged, the exhaust gas flowing through an exhaust passage is more easily drawn back into a cylinder due to a negative pressure produced in the intake passage. Accordingly, the amount of exhaust gas that remains in the combustion chamber increases, which reduces the efficiency of combustion that takes place in the combustion chamber. Accordingly, when the engine load is increased while the internal combustion engine is running at a low speed, if the duration of the intake valve is extended to increase in intake air amount, the combustion efficiency may be reduced rather than being enhanced and engine stalling may be caused.